Warship Island, also called "Battleship Island, " is an anime-exclusive location visited by the Straw Hat Pirates in the Warship Island Arc. The Sennenryu, a breed of dragons thought to have gone extinct that lived for a thousand years, come to this island that is shaped like a battleship to die and be reborn.
One Piece Episode 55 "The Holy Beast! Apis' Secret and the Legendary Island"
A little girl named Apis narrowly escapes from the fleet of Branch 8 Marine fleet, and boards on a little raft. Usopp notices the seagull-filled raft that Apis is on. Usopp and Sanji believe this to be a school of fish, so they ask Luffy to capture them. After searching for Apis, Luffy accidently knocks Zoro off the boat, leaving them with only a very bewildered and shocked Apis.
They eventually learn that she escaped Marine Branch 8 and is now evading Commodore Nelson. After agreeing to bring her to Warship Island, the Straw Hats come under attack from Branch 8 not long after. After eluding the Marines, the Straw Hats realize they are in the Calm Belt, where they discover enormous sea kings.
After narrowly avoiding the Sea Kings and the Calm Belt, they head towards Warship Island. Once they arrive at Warship Island, they learn that Apis has a Devil Fruit talent that enables her to communicate with animals. She used it to make friends with a dying Sennenryu, a sort of dragon thought to be extinct.
Warship Island has a place called The Dragon's Nest. This vast piece of ground emerges from the ocean once every thousand years. The Sennenryu congregate here.
Apis is a little girl residing in Warship Island with her grandfather Bokuden.
Bokuden is Apis’s grandfather, who lives with her in Warship Island.
The Sennenryu who accompanied the Straw Hat Pirates was known as Grandfather Ryu (along with Apis). Ryu was an ancient and frail dragon that could no longer fly properly.
A Sennenryu, meaning "Thousand Year Dragon" or "Millennial Dragon" come from Warship Island's mostly submerged Dragon's Nest.
There are two localities in Japan with the moniker "Gunkanjima," or "Battleship Island," therefore, the island may have been modelled on one of those locations. Hashima Island was given this moniker since the American Navy torpedoed it during World War II because of how much it resembled a battleship. This moniker was also given to Mitsukejima because of its shape.
The Warship Island Arc was a fantastic demonstration of what a One Piece filler arc could do. Some of its components even became canon in the show, including Luffy's intriguing capacity for understanding mythical creatures.
The Warship Island arc can be skipped if the viewers wish to since it has very little to nothing to do with the canon stories.
Warship Island arc is shown in 8 episodes in the One Piece series from Episode 54-61.